Read this webpage:
tails.boum.org
With sincere respect, you are trying to use Tails in a way that it was not designed to do. Tails exists to avoid leaving traces of your past activity. The threats include well-funded adversaries with state-of-the-art forensics labs.
NOT LIKELY TO HAPPEN:
To fix this issue, you may need to build your own special version of Tails that includes the desired password manager built-in, configured to save its data on the encrypted partition. That exposes some security issues which you may want to avoid.
What is your threat model? Can you share why Tails and not an ordinary Linux distro that boots and runs from an fully encrypted drive?